1. Field of the Invention
The invention relates to the production of methane from carbon monoxide. More particularly, it relates to a methanation process capable of effectively utilizing dilute carbon monoxide-containing gas streams also containing appreciable quantities of hydrogen.
2. Description of the Prior Art
The catalytic hydrogenation of carbon monoxide to form methane is one of the most well known and established hydrogenation reactions. This reaction, which is: EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O, (1)
utilizes a synthesis gas, as from the gasification of coal with oxygen and steam, that is treated to provide a desired H.sub.2 /CO ratio and to remove excess CO.sub.2 and deleterious impurities such as sulfur compounds. As the H.sub.2 /CO ratio of the raw synthesis gas is substantially below the necessary minimum ratio of 3/1, at least a portion of the carbon monoxide in the gas is generally first reacted with steam, over an iron or other suitable catalyst in the well-known "water shift" reaction, as follows: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2. (2)
Excess CO.sub.2 in the gas stream is removed by conventional means, such as by treatment with alkaline absorbents. Sulfur impurities are also removed to substantially under 5 ppm, e.g., to less than about 1 ppm, preferably to less than 0.2 ppm, to protect the methanation catalyst from poisoning by such sulfur impurities.
The water shift reaction normally does not go to completion, with the equilibrium determined by the reaction temperature and other operating conditions limiting the degree of completeness of the reaction. The desired H.sub.2 /CO ratio is obtained, to achieve maximum utilization of the available CO and hydrogen, either by very careful choice and control of the processing conditions or by the treatment of a portion of the raw synthesis gas to produce a H.sub.2 /CO ratio substantially in excess of 3/1 and blending the treated gas with the untreated portion to produce the desired H.sub.2 /CO ratio.
One variation of the latter approach is disclosed in the Muller patent, U.S. Pat. No. 3,854,895, in which the primary gas from coal gasification is divided into two streams, one of which is subjected to water shift and subsequent methanation stages, the untreated stream being added to said treated gas successively during said methanation stages. Numerous prior art techniques also exist, it should be noted, for the production of methane from other gases containing hydrogen and carbon oxides. Humphries et al, U.S. Pat. No. 3,511,624, for example, relates to the two-stage production of a gas containing a high proportion of methane from a reaction mixture comprising hydrogen, carbon monoxide and dioxide, steam and at least 25% by volume methane.
Despite the established nature of the major steps in the known techniques for the gasification of coal and in the methanation of the resulting synthesis gas, improved processes for the production of methane are urgently needed to enhance the overall economics of methane production and/or to enable its production from carbon-monoxide-containing gas streams that cannot presently be used in a commercially feasible manner for the production of methane. This need is highlighted by the diminishing supply of natural gas and the recognized need to develop economical supplies of synthetic natural gas to meet existing and anticipated requirements for low-cost, high BTU gaseous heating fuels.
In the processing of synthesis gas obtained by the gasification of coal with oxygen and steam, pretreatment by the catalytic water shift reaction is a major processing step prior to methanation. This necessary adjustment of the H.sub.2 /CO ratio adds, of course, to the overall processing costs and necessarily reduces the amount of carbon monoxide available for conversion to methane. In addition, gas streams containing a low concentration of carbon monoxide and/or a high concentration of inert gases are generally not suitable for methanation purposes because of the costs associated with the concentration of the carbon monoxide, as by cryogenic or absorption means. For example, the effluent from the underground gasification of coal with air is not a suitable feed gas for conventional methanation techniques because of the high proportion of inert gases, i.e., nitrogen, in said effluent. Similarly, the effluent from blast furnace operations contains a high proportion of nitrogen and is not suitable for the economic production of methane because of the prohibitive cost of concentration of the carbon monoxide content thereof.
Conventional processing techniques, in addition, are known to have particular operating difficulties, the overcoming to which tends to shift the equilibrium and reduce the yield of desired methane product or tends to reduce the overall efficiency of the production reaction. The Muller patent referred to above, i.e., U.S. Pat. No. 3,854,895, thus discloses that the formation of free carbon by the Boudouard reaction is promoted by an increase of CO in the reaction mixture, leading the prior art to employ excess hydrogen and to obtain not pure methane but a mixture of methane and hydrogen. The above-mentioned Humphries et al patent, U.S. Pat. No. 3,511,624, likewise refers to said Boudouard reaction: EQU 2CO.fwdarw.CO.sub.2 +C (3)
and discloses the known use of steam to react with a portion of the CO in the gas stream per reaction (2) above to assure the presence of sufficient CO.sub.2 to prevent the Boudouard reaction from moving to the right and causing carbon deposition, the process including the removal of the resulting carbon dioxide and any remaining steam from the mixture. While the problem of undesired carbon deposition is thus avoided in the art, the necessary adjustments to achieve this result create a further incremental limitation on the processing economy and flexibility of the prior art techniques for methane production. For practical commercial operations, it is highly desirable in the art that, without such limitations, a methanation process be developed that can be operated on a continuous cyclic basis without undue loss of catalyst efficiency and without a need for continuous catalyst regeneration as a necessary step of the cyclic methanation process.
It is an object of the invention, therefore, to provide an improved process for the production of methane.
It is another object of the invention to provide a process for the low-cost production of methane from carbon monoxide-containing gas streams.
It is another object of the invention to provide a process for the production of methane from dilute carbon monoxide-containing gas streams without the necessity for preliminary concentration of the carbon monoxide in said gas streams.
It is another object of the invention to provide a process for the enhanced catalytic production of methane from carbon monoxide-containing gas streams on a cyclic basis without requiring catalyst regeneration as a necessary step of the cyclic process.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.